Engine brakes or retarders are used to assist and supplement wheel brakes in slowing heavy vehicles, such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today's heavy vehicles.
Problems with existing engine braking systems include high noise levels and a lack of smooth operation at some braking levels resulting from the use of less than all of the engine cylinders in a compression braking scheme. Also, existing systems are not readily adaptable to differing road and vehicle conditions. Still further, existing systems are complex and expensive.
Known engine compression brakes convert an internal combustion engine from a power generating unit into a power consuming air compressor.
U.S. Pat. No. 3,220,392 issued to Cummins on Nov. 30, 1965, discloses an engine braking system in which an exhaust valve located in a cylinder is opened when the piston in the cylinder nears the top dead center (TDC) position on the compression stroke. An actuator includes a master piston, driven by a cam and pushrod, which in turn drives a slave piston to open the exhaust valve during engine braking. The braking that can be accomplished by the Cummins device is limited because the timing and duration of the opening of the exhaust valve is dictated by the geometry of the cam which drives the master piston and hence these parameters cannot be independently controlled.
In conjunction with the increasingly widespread use of electronic controls in engine systems, braking systems have been developed which are electronically controlled by a central engine control unit which optimizes the performance of the braking system.
U.S. Pat. No. 5,012,778 issued to Pitzi on May 7, 1991, discloses an engine braking system which includes a solenoid actuated servo valve hydraulically linked to an exhaust valve actuator. The exhaust valve actuator comprises a piston which, when subjected to sufficient hydraulic pressure, is driven into contact with a contact plate attached to an exhaust valve stem, thereby opening the exhaust valve. An electronic controller activates the solenoid of the servo valve. A group of switches are connected in series to the controller and the controller also receives inputs from a crankshaft position sensor and an engine speed sensor.
U.S. Pat. No. 5,255,650 issued to Faletti et al. on Oct. 26, 1993, and assigned to the assignee of the present application, discloses an electronic control system which is programmed to operate the intake valves, exhaust valves, and fuel injectors of an engine according to two predetermined logic patterns. According to a first logic pattern, the exhaust valves remain closed during each compression stroke. According to a second logic pattern, the exhaust valves are opened as the piston nears the TDC position during each compression stroke. The opening position, closing position, and the valve lift are all controlled independently of the position of the engine crankshaft.
U.S. Pat. No. 4,572,114 issued to Sickler on Feb. 25, 1986, discloses an electronically controlled engine braking system. A pushtube of the engine reciprocates a rocker arm and a master piston so that pressurized fluid is delivered and stored in a high pressure accumulator. For each engine cylinder, a three-way solenoid valve is operable by an electronic controller to selectively couple the accumulator to a slave bore having a slave piston disposed therein. The slave piston is responsive to the admittance of the pressurized fluid from the accumulator into the slave bore to move an exhaust valve crosshead and thereby open a pair of exhaust valves. The use of an electronic controller allows braking performance to be maximized independent of restraints resulting from mechanical limitations. Thus, the valve timing may be varied as a function of engine speed to optimize the retarding horsepower developed by the engine.
A number of patents disclose the use of a turbocharger with an engine operable in a braking mode. For example, Pearman et al. U.S. Pat. No. 4,688,384, Davies et al. U.S. Pat. No. 5,410,882 and Custer U.S. Pat. No. 5,437,156 disclose compression release engine braking systems wherein the intake manifold pressure of the engine is controlled so that excessive stresses in the engine and engine brake are prevented. The Pearman et al. and Custer patents disclose the use of pressure release apparatus connected directly to the intake manifold whereas the system disclosed in the Davies et al. patent retards the turbocharger in any of a number of ways, such as by restricting the flow of exhaust gas to or from the turbocharger or by controlling the exhaust gas flow to bypass the turbocharger.
Meneely U.S. Pat. No. 4,932,372 likewise discloses the use of a turbocharger with an engine operable in a braking mode. In addition to the mechanism for opening each exhaust valve of each cylinder of the engine near top dead center of each compression stroke, the Meneely apparatus includes means for increasing the pressure of gases in the exhaust manifold sufficiently to open exhaust valves of other cylinders on the intake stroke after each exhaust valve on the compression stroke is so opened. Such means comprises a device within the turbocharger for diverting the exhaust gases to a restricted portion of the turbine nozzle, thereby increasing the pressure of gases directed onto the turbine blades of the turbocharger and causing back pressure to be developed in the exhaust manifold.
In each of the foregoing systems, controllability over engine braking levels is accomplished by varying boost magnitude alone inasmuch as the timing and duration of exhaust valve opening events are preset by establishing the lash between the exhaust valve actuator and the exhaust valve accomplished by varying boost magnitude alone inasmuch as the timing and duration of exhaust valve opening events are preset by establishing the lash between the exhaust valve actuator and the exhaust valve crosshead. Accordingly, only a limited degree of variability in braking magnitude can be accomplished.